1. Field of the Invention
The present invention relates to a stabilizer for preparing polymer polyols, and more particularly to a stabilizer for preparing polymer polyols, which has high molecular weight and high functionality.
2. Background of the Related Art
Polymer polyol compositions, which are used in the preparation of polyurethane foams, are known materials and were first disclosed in U.S. Pat. Nos. 3,383,351 and 3,304,273. Such compositions are prepared by polymerizing one or more unsaturated monomers (styrene monomer, acrylonitrile, etc.), dissolved in polyol, in the presence of a free radical catalyst. Thus, the produced polymer particles are stably dispersed in polyol liquid, and polyurethane foams prepared from this kind of polymer polyol have high hardness and physical properties, compared to those of polyurethane foams prepared from general polyols.
Polyurethane foams prepared from polymer polyols are mainly used in the field of soft slab stock and soft molds. The soft slab stock is used in cushions, including mattresses, furniture, sofas, shoes and package buffers, and the soft mold is used in various applications, including automobile sheets, bumpers, head liners, instrument panels and sun visors.
Initially commercialized polymer polyols are prepared by polymerizing either acrylonitrile or a styrene/acrylonitrile mixture having low styrene content in polyol. They have high viscosity, yellow color, and scorch in the resulting foam products. Due to this problem, such polymer polyols could be used only in some limited applications.
In order to solve these problems, a polymer polyol prepared using a styrene/acrylonitrile mixture having high styrene content (65-75%) was developed. However, there are problems in that the increase in the styrene content leads to a great reduction in the dispersion stability of the polymer polyol, and thus the amount of solid mass produced in a filtration process is increased. Also, because the viscosity of the polymer polyol itself has a tendency to increase rapidly, it is very difficult to prepare a polymer polyol having the low viscosity and high solid content demanded in the market.
U.S. Pat. No. 4,208,314 discloses low-viscosity polymer polyols prepared using a styrene/acrylonitrile mixture. The polymer polyols of this patent are characterized by high styrene content and can provide polyurethane foams having reduced scorch. Also, U.S. Pat. No. 4,148,314 discloses a method of preparing a polymer polyol, having high dispersion stability and filterability, by adding a small amount of prepolymerized polymer polyol to a polymer polyol polymerization reaction.
In order to manufacture polyurethane having further improved physical properties, efforts to improve the properties of polymer polyols have been made. Particularly, in order to prepare a low-viscosity polymer polyol, having high polymer content together with high dispersion stability, a method of introducing a NAD (non-aqueous dispersant) stabilizer was suggested.
The NAD stabilizer serves to introduce a small amount of unsaturation into polyol and contributes to increasing the dispersion stability of solids, which are produced in the preparation of polymer polyols. U.S. Pat. Nos. 3,652,639, 3,823,201 and 3,850,861, GB Patent No. 1,126,025 and Japanese Patent Nos. 52-80919 and 48-101494 all relate to methods which use the NAD stabilizers. The concept of using the stabilizer in polymerization is well known in the prior art and is old technology as disclosed in Dispersion Polymerization in Organic Media, edited by K. E. J. Barrett, John Wiley Sons, Copyright 1975.
As relatively recent patents which employ this technology, U.S. Pat. Nos. 4,454,255 and 4,458,038 employ, as a stabilizer, a reaction product of polyol with a compound having reactive unsaturation, such as maleic anhydride or fumaric acid. Also, U.S. Pat. No. 4,460,715 discloses preparing a stabilizer using acrylate or methacrylate as reactive unsaturation.
U.S. Pat. No. 4,550,194 discloses the preparation of a stabilizer, which comprises reacting polyol (prepared from pentaerythritol having an OH functionality of 4) with maleic anhydride in the presence of calcium naphthenate and cobalt naphthenate as a catalyst, and reacting the produced carboxylic acid structure with an alkylene oxide such as ethylene oxide or propylene oxide. Similarly, in U.S. Pat. No. 4,997,857, the process of introducing unsaturation into polyol by maleic anhydride, and reacting the polyol with an alkylene oxide, is almost similar to that of U.S. Pat. No. 4,550,194, but high-molecular weight polyol having an OH functionality greater than 4, for example, sorbitol having an OH functionality of 6, is used as the polyol of a stabilizer.
The above-described stabilizers have advantages in that polymer polyols prepared using these stabilizers have low viscosity, and the resulting polyurethane has good physical properties. Despite these advantages, these stabilizers have a disadvantage in that they require a process of increasing the viscosity and molecular weight thereof either by heating for a long time or by adding diisocyanate coupling agents, such as toluene diisocyanate or methylene diisocyanate, after the stabilizers are prepared by allowing polyol to react with maleic anhydride and alkylene oxide.
Thus, there has been a continued demand for a method, which can prepare a high-viscosity, high-molecular-weight stabilizer without adding coupling agents or using a lengthy heating process.